TECHNICAL FIELD

The aspects of the present disclosure relate generally to the field of filters. More particularly the present invention relates to a filter assembly.

BACKGROUND Filters are circuits that are used in communication systems to compensate for disturbances such as e.g. interference, caused by the nature of the transmission media between sender and receiver. Filters remove any undesired communication signal components and/or enhance the desired communication signal components.

Radio Frequency (RF) filters and Microwave filters represent a class of filters, designed to operate on signals in the Megahertz to Gigahertz frequency ranges. This frequency range is the range used by most broadcast radio, television and wireless communication systems such as e.g. cellular communication systems, Wi-Fi, WiMax, LTE, etc. Thus, most wireless communication devices will include some kind of filter assembly filtering transmitted and/or received signals. Further, filter assemblies are also used in radio interface communication nodes such as e.g. a radio antenna of a broadcast radio system, a TV broadcasting antenna of a television system and a radio base station of a cellular telephone system. Such filters are commonly used as building blocks for duplexers and diplexers to combine or separate multiple frequency bands.

Presently, two technologies are most widely used for radio base station front end filters. These filter technologies, coaxial filters and ceramic filters each consist of a number of resonators, coupled together providing a proper transfer of desired signals and rejection of undesired signals. A driving force within the development of filters today is the issue of size. Smaller filters allow smaller electronic devices that the filters are installed in. This reduces the space for the equipment required for storage, shipment and installation at the customer's site. Thus, it is desirable to provide small and compact filters with sufficient performance .

One way of measuring the performance of a filter is by their Quality or Q factor. A filter has a high Q factor if the filter is capable of selecting or rejecting a range of frequencies that is narrow in comparison to the centre frequency. The Q factor represents a relationship between a stored and dissipated energy in a resonant circuit. The Q factor may be defined as the ratio of centre frequency divided by 3 dB bandwidth. The pass band loss of a filter is inversely proportional to unloaded Q.

Classical coaxial resonators with a metal centre conductor are inexpensive to manufacture and provide adequate performance, such as e.g. a Q factor of 2500 with the cavity volume of 22*22*22 mm ³ or a Q factor of 4300 with the cavity volume of 37*37*37 mm at 2 GHz frequency, and low manufacturing cost. These classical coaxial resonators are fairly scalable in size, such as e.g. coaxial resonator lengths from 15 mm to 100 mm depending on the frequency used. The coaxial resonators may make use of high-dielectric constant materials instead of metal to reduce their overall size and thus enable the scalability. One disadvantage with ordinary coaxial resonators may be the limited power handling capability, which is caused by the small gap between the resonator and the tuning element.

The ceramic resonators, such as e.g. ceramic Transverse Electric (TE) TEOld single mode resonators, are used for high performance, such as e.g. a Q factor of 10, 000 and above. Ceramic resonators provide higher performance compared to classical coaxial resonators or waveguides with a maximum Q factor being less than 10,000 at 2 GHz frequency . Ceramic resonators are made of high-stability piezoelectric ceramics, generally lead zirconium titanate (PZT) which functions as a mechanical resonator. Ceramic resonators for TE and TM mode are made of a material compound of e.g. oxygen (0) , barium (Ba) , titanium (Ti) , zinc (Zn) , neodymium (Nd) , and lanthanum (La) . The TEOld single mode ceramic resonators require rather large cavities. At 1.9 GHz frequency the Q factor is about 3,200 for a coaxial resonator when cavity is about 30*30 mm (height*diameter) . The size of the TEOld single mode ceramic puck resonator is about 27.5*10 mm (height*diameter) in the same cavity as above. Smaller cavity size with TEOld mode at 1.9 GHz frequency is not possible, because it is then necessary to increase the puck diameter.

Furthermore, several manufacturers have used dielectric resonators such as e.g. Transverse Magnetic (TM) single mode resonators, as radio base station front end filters. TM resonators enable considerable size reduction compared to metal resonators without loss of performance relative to a metal coaxial resonator. A typical TM single mode resonator saves 20-50% volume, depending on the resonant frequency and dielectric constant of the ceramic, compared to a coaxial metal resonator of the same unloaded Quality factor.

Other technologies which also have been used for radio base station front end filters are very complex shaped TM dual mode resonators and TM triple mode resonators. The size reduction with this technology is about 30-80% compared to a coaxial metal resonator of the same unloaded Quality factor and of the same resonant frequency.

The application of TM mode is when both resonator ends are grounded. A commonly used method for grounding both resonator ends is e.g. soldering the dielectric rods directly to the filter housing and filter cover. A problem with known solutions, using soldering to attach the TM mode dielectric rods to the filter housing and/or the lid, is that once the dielectric rod has been assembled and soldered, the dielectric rod cannot be replaced. To replace one single dielectric rod in a filter, at least one end of all the other soldered dielectric rods in the filter must be de-soldered, such as e.g. de-soldered from the lid side. In practice, this is, however, not possible, due to the fact that the conductive plating material, such as e.g. the silver plating, at the dielectric rod ends will only be good for one soldering operation, thus the dielectric rod is not replaceable.

In another known TM mode filter the dielectric rod is provided with a conductive element in the form of a washer placed at a first rod end. The washer is in conductive contact with an outwardly facing recess in a first wall of the filter chassis (could be the cover) sized and shaped for receiving and contacting the washer. The recess is threaded and a first threaded disk is in threaded engagement with the recess to press the washer into the recess to ensure reliable contact between the chassis and the washer. In the centre of the recess there is an opening that allows the dielectric rod to be inserted into the filter cavity. The dielectric rod is provided with a second conductive element in the form of a second threaded disk at the opposite second rod end. The second threaded disk is in threaded engagement with a threaded through-going hole in a second wall of the filter chassis that is on the opposite side of the filter cavity relative to the first wall. This construction allows the dielectric rod to be replaced. However, this construction will cause small metal particles to find their way into the filter cavity when the second threaded disk engages the threaded through-going hole in the second wall. This will result in a lesser size reduction and a non- optimized PIM (Passive intermodulation) since the bottom part/washer of the dielectric rod is screwed into the chassis and this causes rotation of the silver plated washer contact surface against silver plated chassis surface. This frictional movement between the silver plated parts will release silver particles and be a PIM source. Further, by rigidly securing the rod at both ends to the filter chassis, inaccuracies in alignment of the two ends due to production tolerances or due to thermal expansion will expose the dielectric rod to forces that may lead to cracks in the dielectric rod. Since ceramic dielectric rods are fragile other solutions to secure a dielectric rod in a filter cavity in a replaceable way are not viable or ideal.

SUMMARY It is an object of the invention to provide a filter assembly that overcomes or at least reduces the above problems .

The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures .

According to a first aspect the above and further objects and advantages are obtained by a filter assembly comprising: a filter chassis defining a filter cavity, the filter chassis comprising a wall, the wall having an inner side and an outer side, the inner side of the wall defining a border of the filter cavity, at least one dielectric rod arranged in the filter cavity, the dielectric rod extending between a first end and a second end, a first conductive element conductively secured to the first end of the dielectric rod, a second conductive element conductively secured to the second end of the dielectric rod, the first conductive element being secured to the wall, a conductive membrane conductively secured to the second conductive element, and the conductive membrane being conductively secured to the filter chassis.

Since the dielectric rod is connected at its first end to a wall of the cavity with its second end of the dielectric rod being connected to the relatively flexible conductive membrane the dielectric rod is only rigidly secured at its first end with the second end of the dielectric rod being relatively free to assume a position dictated by the connection of the first end to the wall. Thus, the dielectric rod is relatively rigidly suspended from the wall from its first end and relatively loosely suspended from the conductive membrane from its opposite second end, thus avoiding stress in the dielectric rod from being rigidly suspended from two, possibly not perfectly aligned, ends/suspension points. This solution avoids bending forces on the dielectric rod and resulting stress in the dielectric rod . In a first possible implementation of the first aspect the first conductive element is rigidly secured to the wall of the filter cavity.

In a second possible implementation of the first aspect or any implementation thereof the second conductive element is free from any rigid connection to the chassis. Hence, at his second end, the dielectric rod is conductively connected to the filter chassis only via the second conductive element and the (flexible) conductive membrane. In a third possible implementation of the first aspect or any implementation thereof the second conductive element is mechanically connected to the filter chassis by the conductive membrane only. In a fourth possible implementation of the first aspect or any implementation thereof the conductive membrane is a thin metal sheet.

In a fifth possible implementation of the first aspect or any implementation thereof the conductive membrane is an aluminum sheet with a thickness between (approximately) 0.2 mm and (approximately) 0.3 mm.

In a sixth possible implementation of the first aspect or any implementation thereof the chassis is a metal chassis and wherein the metal of the chassis is substantially the same metal or preferably is the same metal as the metal of the conductive membrane. Thus, differences in thermal expansion between the chassis and the conductive membrane can be minimized or avoided.

In a seventh possible implementation of the first aspect or any implementation thereof the first conductive element has a first axially facing contact surface in contact with the wall. In an eighth possible implementation of the first aspect or any implementation thereof the second conductive element has a second axially facing contact surface in contact with the conductive membrane.

In a ninth possible implementation of the first aspect or any implementation thereof the filter assembly further comprises a cover partially covering the filter cavity with a portion of the conductive membrane placed between the cover and the chassis. Thus, a good electrical contact between the chassis and the conductive membrane is achieved.

In a tenth possible implementation of the first aspect or any implementation thereof the cover is provided with an insertion opening shaped and sized to allow the electric rod together with the first conductive element and the second conductive element to be inserted through the insertion opening . In an eleventh possible implementation of the first aspect or any implementation thereof the cover is configured to press the conductive membrane onto a surface of the chassis. Thus, a good electrical contact between the chassis and the conductive membrane is achieved.

In a twelfth possible implementation of the first aspect or any implementation thereof the wall is provided with a first connection opening and wherein a first fastener that is applied from the outer side of the wall connects to the first conductive element. Thus, the first end of the dielectric rod can be secured to the wall without risking that metal parts released by relative movement between metal parts, such as the chassis and the fastener, due to rotation for establishing threaded connection end up in the filter cavity and deteriorate the performance of the filter assembly.

In a thirteenth possible implementation of the first aspect or any implementation thereof the first conductive element and/or the fastener extend at least partially into the first connection opening.

In a fourteenth possible implementation of the first aspect or any implementation thereof the first fastener and the first conductive element are in threaded engagement.

In a fifteenth possible implementation of the first aspect or any implementation thereof the first fastener is brought in threaded engagement with the first conductive element by rotation of the first fastener with the first conductive element being stationary. Thus, any torsion forces on the fragile dielectric rod can be avoided.

In a sixteenth possible implementation of the first aspect or any implementation thereof the first conductive element has a first engagement portion that protrudes through the first connection opening in the wall.

In a seventeenth possible implementation of the first aspect or any implementation thereof the first fastener engages the first engagement portion that protrudes through the first connection opening in the wall. In an eighteenth possible implementation of the first aspect or any implementation thereof a portion of the wall surrounding the first connection opening is arranged between the first axially facing contact surface and a third axially facing contact surface of the first fastener.

In a nineteen possible implementation of the first aspect or any implementation thereof the first engagement portion comprises an externally threaded cylindrical portion and the first fastener comprises an internally threaded hole and wherein the first fastener is in threaded engagement with the first engagement portion. In a twentieth possible implementation of the first aspect or any implementation thereof the conductive membrane is provided with a second connection opening and wherein a second fastener that is applied from an outer side of the conductive membrane connects to the second conductive element

In a twenty-first possible implementation of the first aspect or any implementation thereof the second conductive element has a second engagement portion that projects through a hole in the metal membrane.

In a twenty-second possible implementation of the first aspect or any implementation thereof the second fastener and the second conductive element are in threaded engagement. In a twenty-third possible implementation of the first aspect or any implementation thereof the second fastener is brought in threaded engagement with the second conductive element by rotation of the second fastener with the second conductive element being stationary.

In a twenty-fourth possible implementation of the first aspect or any implementation thereof the second fastener engages the second engagement portion that protrudes through the second connection opening in the metal membrane.

In a twenty-fifth possible implementation of the first aspect or any implementation thereof the conductive membrane is arranged between the second axially facing contact surface and a fourth axially facing contact surface of the second fastener.

In a twenty-sixth possible implementation of the first aspect or any implementation thereof the second engagement portion comprises an externally threaded cylindrical portion and the second fastener comprises an internally threaded hole and wherein the second fastener is in threaded engagement with the second engagement portion. In a twenty-seventh possible implementation of the first aspect or any implementation thereof the first conductive element is conductively secured to the first end by solder, by conductive adhesive or by conductive glue. In a twenty-eight possible implementation of the first aspect or any implementation thereof the second conductive element is conductively secured to the second end by solder, by conductive adhesive or by conductive glue.

In a twenty-ninth possible implementation of the first aspect or any implementation thereof the dielectric rod is a hollow cylinder with a cylindrical hollow interior.

In a thirtieth possible implementation of the first aspect or any implementation thereof the first conductive element and/or the second conductive element is provided with a through-going hole that is axially aligned with the cylindrical hollow interior of the dielectric rod.

These and other aspects of the invention will be apparent from the examples and the embodiment ( s ) described below.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed portion of the present disclosure, the invention will be explained in more detail with reference to the example embodiments shown in the drawings, in which:

Fig. 1 is a sectional view of an example embodiment of a filter assembly showing a single dielectric rod,

Fig. 2 is a sectional view of a dielectric rod used in the filter assembly of Fig. 1,

Fig. 3 is a sectional view of another example embodiment of a filter assembly with a plurality of dielectric rods,

Fig. 4 is an elevated cutaway view of the filter assembly of Fig. 3, and Fig. 5 is a top view of the filter assembly of Fig. 3, DETAILED DESCRIPTION

Fig. 1 shows a cross-section of a filter assembly 1 according to an example embodiment. The filter assembly 1 includes a filter chassis 2 and at least one dielectric rod 7. The filter assembly 1 may comprise a plurality of dielectric rods 7. However, only one dielectric rod 7 is depicted in Fig. 1. The filter assembly 1 may be a TM single mode resonator. Fig. 2 shows a dielectric rod 7 in greater detail .

The filter chassis 1 defines a filter cavity 3 that can be covered with a lid or cover 14. The filter chassis 2 and cover 1 may be made of e.g. silver plated copper on aluminum. The filter cavity 3 is defined by a number of walls that are part of the filter chassis 2 and by a conductive membrane 10. The conductive membrane 10 is placed over an open side of the filter cavity 3 and the cover 14 is placed over the conductive membrane 10. The conductive membrane 10 is sandwiched between a surface of the chassis 2 and a surface of the cover 10 in order to establish a reliable electric connection between the filter chassis 2 and the conductive membrane 10. In an embodiment the conductive membrane 10 is a thin metal sheet. An example of such a thin metal sheet suitable as conductive cover is an metal sheet with a thickness between approximately 0.2 mm and approximately 0.3 mm. The conductive membrane can be made of aluminum or an aluminum alloy. The conductive membrane 10 can be an aluminum or aluminum alloy sheet with a thickness between approximately 0.2 mm and approximately 0.3 mm. Other suitable metals or materials with good conductive properties can also be used for the membrane. The conductive membrane 10 can be layered with layers having different properties and being made of different materials.

The chassis 2 and conductive membrane 10 can in an embodiment be made from the same or substantially the same metal in order to avoid any differences in thermal expansion of the two items.

One of the walls 4 serves to secure one end of the dielectric rod 7 rigidly to the filter chassis 2. In this example embodiment the wall 4 is on the opposite side of the cavity relative to the conductive membrane 10. The wall 4 has an inner side 5 facing the filter cavity 3 and an outer side 6 facing away from the filter cavity 3. The inner side 5 of the wall 4 defines a border of the filter cavity 3.

At least one dielectric rod 7 is arranged inside the filter cavity 3. The dielectric rod 7 extends between a first end and a second end. The dielectric rod 7 extends from the wall 4 to the conductive membrane 10, the latter being disposed on the opposite side of the filter cavity 3 relative to the wall 4. A first conductive element 8 is conductively secured to the first end of the dielectric rod 7. A second conductive element 9 is conductively secured to the second of the dielectric rod 7. The first and/or second conductive 8,9 elements may be coated for improving electric contact, such as e.g. by silver plating. In the shown embodiment the dielectric rod 7 is a hollow cylinder with a cylindrical hollow interior 22, but it is understood that the dielectric rod 7 can also be a solid rod (is not shown) .

In an embodiment, the first conductive element 8 is conductively secured to the first end of the dielectric rod 7 by solder, by conductive adhesive or by conductive glue. In an embodiment, the second conductive element 9 is conductively secured to the second end of the dielectric rod 7 by solder, by conductive adhesive or by conductive glue.

The first conductive element 8 is secured to the wall 4. The first conductive element 8 is preferably rigidly secured to the wall 4 of the filter cavity, so that the position of the dielectric rod 7 is defined by the connection between the first conductive element 8 and the wall 4.

The second conductive element 9 is conductively secured to the conductive membrane 10. In an embodiment the second conductive element 9 is mechanically connected to the filter chassis 2 by the conductive membrane 10 only. The second conductive element 9 is not connected in any other way to the filter chassis 2. The second conductive element 9 is not connected to the cover 13 either. The connection between the second conductive element 9 and the conductive membrane does not form a rigid connection for the second conductive element 9 because conductive membrane is relatively flexible. Thus, the second conductive element 9 is free from any rigid connection to the filter chassis 2. Consequently, even if there should be a slight misalignment due to e.g. production tolerances or thermal expansion, there will not be any force of significance applied by the membrane to the second conductive element 9 and thus there will not be any resulting stress in the (relatively fragile) dielectric rod 7.

In an embodiment the first conductive element 8 and the second conductive element 9 are metal objects including a cylindrical portion with an annular flange.

The first conductive element 8 has a first axially facing contact surface 11 in contact with the wall 4, preferably, the inner side 5 of the wall 4. The first axially facing contact surface 11 is in an embodiment part of an annular flange of the first conductive element 8.

The second conductive element 9 has a second axially facing contact surface 12 in contact with the conductive membrane 10, preferably the surface of the conductive membrane 10 facing the filter cavity 3. The second axially facing contact surface 12 is in an embodiment part of an annular flange of the second conductive element 9.

The cover 13 partially covers the filter cavity 3 with a portion of the conductive membrane 10 placed between the cover 13 and the chassis 2. Essentially, the cover 13 covers all of the filter cavity, except for insertion opening 14. The conductive membrane 10 is sandwiched between a side of the filter chassis 2 and a side of the conductive membrane 10. The cover 13 is configured to press the conductive membrane 10 onto a surface (side) of the chassis 2 e.g. by means of suitable fasteners e.g. screws screwed through the cover 13 into the chassis 2.

The insertion opening 14 is shaped and sized to allow the dielectric rod 7 together with the first conductive element 8 and the second conductive element 9 to be inserted through the insertion opening 14.

The wall 4 is provided with a first connection opening 15. The first connection opening 15 can e.g. be a simple round through-going hole. A first fastener 16 is applied from the outer side 6 of the wall 4 and connects to the first conductive element 8. The first conductive element 8 has a first engagement portion 17 that protrudes through the first connection opening 15 in the wall 4. The first engagement portion 17 can e.g. be a cylindrical portion. The first fastener 16 engages the first engagement portion 17 that protrudes through the first connection opening 15 in the wall 4. The first fastener 16 can e.g. be a nut with an internally threaded hole.

A portion of the wall 4 surrounding the first connection opening 15 is sandwiched between the first axially facing contact surface 11 and a third axially facing contact surface 18 of the first fastener 16. Thus, the first conductive element 8 is securely and rigidly attached to the wall 4. In an embodiment the first engagement portion 17 is provided with an externally threaded cylindrical portion and the first fastener 16 comprises a matching internally threaded hole and the first fastener 16 is in threaded engagement with the first engagement portion 17.

In an embodiment the first fastener 16 is brought in threaded engagement with the first conductive element 8 element by rotation of the first fastener 16 with the first conductive element 8 being stationary.

In an embodiment (not shown) the first fastener 16 extends at least partially into the first connection opening 15. The conductive membrane 10 is provided with a second connection opening 18. A second fastener 19 is applied from an outer side of the conductive membrane 10 and connects to the second conductive element 9. In an embodiment the second conductive element 9 has a second engagement portion 20 that projects through the second connection opening 18 in the metal membrane 10. Preferably, the second fastener 19 and the second conductive element 9 are in threaded engagement. The second fastener 19 is preferably brought in threaded engagement with the second conductive element 9 by rotation of the second fastener 19 with the second conductive element 9 being stationary.

In an embodiment the second fastener 19 engages the second engagement portion 20 that protrudes through the second connection opening 18 in the metal membrane 10. A portion of the conductive membrane 10 is sandwiched between a second axially facing contact surface 12 of the second conductive element 9 and a fourth axially facing contact surface 21 of the second fastener 19.

The second engagement portion 20 is provided with an externally threaded cylindrical portion and the second fastener 19 comprises a matching internally threaded hole. The second fastener 19 can be a nut that is in threaded engagement with the second engagement portion 20.

In an embodiment the first conductive element 8 and/or the second conductive element 9 is provided with a through-going hole 27, 28 that is axially aligned with the cylindrical hollow interior 22 of the dielectric rod 7. In the shown embodiment the through-going hole 28 in the second conductive element 9 is internally threaded with a threaded tuning rod 25 received therein. The tuning rod 25 can be secured after adjustment in its position by a locking nut 26 that engages the external thread of the tuning rod 25.

Fig. 3 is a lengthwise sectional view through a filter assembly 1 according to an example embodiment.

A filter chassis 2 can be provided with one or more filter cavities and a filter cavity can be provided with one or more dialectic rods 7. The dielectric rods 7 can all be arranged in equal orientation in a filter cavity or in different orientations. The dielectric rods 7 may be used to form Transverse Electric filters and/or Transverse Magnetic single mode filters. According to some embodiments a plurality of dielectric rods 7 may be arranged consecutive in parallel into one filter chassis 2 to form a Transverse Electric filter and/or a Transverse Magnetic single mode filter.

According to some embodiments a plurality of dielectric rods 7 may be arranged consecutive to form a Transverse Magnetic dual mode filter. These embodiments comprises a plurality of dielectric rods 7 that may be arranged consecutive such that at least two of the plural of dielectric rods 7 are perpendicular and/or orthogonal to each other into one filter chassis 2.

Fig. 4 is an elevated cutaway view of the filter assembly of Fig. 3 without any dielectric rods 7 installed. Fig. 5 is a top view of the filter assembly of Fig. 3. The invention has been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

The reference signs used in the claims shall not be construed as limiting the scope.